Thanks for your response.
I have text files that contain instructions for my customers. In these text files I have added variables, marked with {…}
Afterwards I use a process to replace {name} with an actual customer name. And {city} with the actual customer city.

I would like Notepad++ to highlight these ‘variables’… So any string starting with { and ending with } should be highlighted with font color red for example.

A lot of things are possible, but the easy way to do this is to use the “Mark” feature (part of find). In the “Find what” field of the Mark tab (in the Find dialog), enter \{.+?\}. Make sure “Search Mode” is set to “Regular expression”. I also would advise checking the “Wrap around” checkbox.

Scotts solution might be the easiest.
Another attempt would be to define your own UDL.
Open UDL (Language->Define your language…)
open tab Operators&Delimiters
In Delimiter 1 style put { in Open: field
and } in Close: field.
Press the Styler button and select the foreground color of your choice.
Save it with a meaningful name and close it.
Your UDL should now be visible under the Language menu.
When ever it is needed, select your user defined language (UDL) and everything between {} should be colored.